In present-day "espresso" coffee makers, the production of froth is due to the expansion, in the infusion leaving the filter, of air bubbles which have been compressed generally to between 10 and 15 bars in the water circuit of the coffee maker. When it passes through the coffee grounds, the pressure of the water decreases, reaching atmospheric pressure on leaving the filtering surface.
This process for the production of froth is the same regardless of the fineness of the ground coffee used. However, it has been noted that the degree of grinding of the coffee affects the quality of the froth by modifying the pressure in the cup.